The present invention generally relates to methods and apparatus for peeling or delaminating a composite sheet structure or laminate containing image media.
In the graphics arts field, it is extremely important that so-called master images of the original objects being reproduced in a prepress operation be free of visually discernible image defects that would impact negatively in the formation of commercially acceptable prints. The master images are, in turn, used in the subsequent formation of either positive or negative copies, depending on the platemaking process employed. Since the final printed image will only be as good as its master image, considerable efforts are undertaken to produce only the highest quality master.
Some conventional approaches in the graphic arts area, such as letterpress and offset lithography, rely on the use of halftones to reproduce continuous tone original images. Halftone techniques employ very small and defined screen dots of varying sizes and arranged in varying spacing patterns to reproduce continuous tone images. The screen dots are relatively small and range in size. If these screen dots are not produced with the desired definition and with the desired spacing patterns, the resultant continuous tone images will be less than the quality desired and thus, render the prints commercially unacceptable. Therefore, it is extremely important that the halftone screen dots and image patterns be precisely formed and processed in a manner which preserves their desired definition and spacing characteristics.
Conventional halftone approaches in prepress printing rely on silver halide photography and can include a graphics art camera of the darkroom type. Development of the latent images on the light sensitive film is achieved through a series of chemical processing steps which include immersing the film in a special developer that converts the silver halide to metallic silver in proportion to the amount of exposure received. Each film type uses special developing agents and other combinations of chemicals and water for fixing and stabilizing the film during processing. Some film processors use lasers for generating images instead of cameras. It will be appreciated that the entire silver halide process suffers several shortcomings because of the chemical storage, handling and disposal concerns it occasions. Moreover, silver halide film is light sensitive and this presents other handling issues in order to protect it.
A recent development in the image forming arts has achieved high quality, high resolution images, such as radiological images of the medical type, without the need for silver halide film and processing techniques along with their attendant equipment and chemical processing issues. In this regard, high quality and high resolution images have been produced by a dry process imaging process which uses laser beams imaging on a thermographic image forming medium. Examples of this kind of thermographic image forming medium are described in commonly assigned International Patent Application No. PCT/US 87/03249 published Jun. 16, 1988, under International Publication Number WO 88/04237; and U.S. Pat. No. 5,200,297. After this laminate has been imaged, it is delaminated by peeling one layer therefrom and having the remaining imaged media relaminated with a protective coating. For a more detailed description of the imaging technique and an associated peeling process, reference is made to commonly assigned U.S. Pat. No.: 5,141,584 to Schuh et al. However, it has been determined that dry peeling the laminate can produce image defects which render a commercially unacceptable product. The peeling defects and their severity are caused by a variety of factors associated with the peeling action itself, and the physical and/or chemical properties of the media. For instance, some of the image defects or artifacts of the resultant peeled images are attributable to the alternating high and low density areas in the image media. For instance, some of these artifacts occur in the vicinity of areas of exposed and unexposed portions that may be both transverse and linear to the web directions. It is, therefore, highly desirable to produce images which are free of these kinds of defects.